1. Field of the Invention
This invention relates to the granulometry field in general. More especially the invention relates to a granulometry process and devices for optical measuring sizes and concentrations of particles in suspension in a gaseous or liquid fluid.
2. Description of the Prior Art
At present in the granulometry field, measuring instruments with the highest performance make use of optical measuring processes. The existing instruments are of two types. Counting instruments requiring the presence of one single particle at a time in a measuring volume belong to a first type. Instruments sample analyzing the light scattering of a sample volume illuminated by a cloud of particles so as to determine a cloud particle size distribution function, belong to a second type.
The instruments of the first type, such as the so-called KNOLLENBERG instruments, use the properties of light scattering by a single particle. The main drawback of these instruments lies in the necessity of the presence of one single particle at a time in the measuring volume. In this way these instruments have limited performances and do not cover a clearly defined size measuring range.
The instruments of the second type use the FRAUNHOFFER approximation or the LORENZ-MIE theory to calculate the light scattering of a cloud of particles. The field of use of these instruments is limited to a voluminal concentration measuring range lying between 10.sup.-9 and 10.sup.-5. The size measuring range depends on the theorical diffusion model employed.
The chief instruments of the second type at present available on the market are those sold under the MALVERN trademark. These instruments make use of the FRAUNHOFFER approximation. The article by E. D. HIRLEMAN and L. G. DODGE entitled "Performance comparison of Malvern instruments laser diffraction drop size analyzers" and published in the 3rd "International Conference on Liquid Atomisation and Spray Systems" (ICLASS 85), pages IVA/3/1 to IVA/3/14, Institute of Energy, London, 1985, indicates their performances.
The particle size distribution is only considered to be very well estimated by the MALVERN instruments for particles having a diameter above 5 .mu.m. Moreover, a considerable drawback to these instruments is their lack of accuracy as soon as the distribution function to be measured cannot be modeled by a simple ROSIN-RAMMLER or log-normal type function. This lack of accuracy is due to the distribution function calculation process requiring a matricial inversion and an estimation by the method of least squares entailing an unstable result.
Moreover, the different instruments available have a form and dimensions which are not suited to measure the sizes and concentrations of particles in a fluid flow. In fact, an unsuitable form and oversized dimensions disturb the flow of the fluid and modify the measurements. These instruments are unadapted to be carried by a moving vehicle, for example by an aircraft, in order to determine the size distribution of droplets in suspension in the air.